What is the missing number?

A man is sent to prison for the first time. At night, the lights in the cell block are turned off, and his cellmate goes over to the bars and yells, "Number twelve!" The whole cell block breaks out laughing. A few minutes later, somebody else in the cell block yells, "Number four!" Again, the whole cell block breaks out laughing.

The new guy asks his cellmate what's going on. "Well," says the older prisoner, "we've all been in this here prison for so long, we all know the same jokes. So we just yell out the number instead of saying the whole joke."

So the new guy walks up to the bars and yells, "Number twenty-nine!" This time the whole cell block rocks with the loudest laughter, prisoners rolling on the floor laughing hysterically.

When the guffaws die down, the bewildered new guy turns to the older prisoner and asks, "How come you guys were laughing so hard this time?"

"Oh," says the older man wiping tears from his eyes, "we'd never heard that one before."